CLSS

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Hydraulic Systems

The industrial hydraulics system is a power transmission system using a fluid to carry the power. Transmission of power with a contained liquid as an essential link in a transmission system can be accomplished by two separate and distinct systems.

Hydrodynamic SystemsHydrodynamic systems depend upon the inertia of the moving fluid to accomplish the desired power transmission function. These systems are called hydrodynamic because of the energy transfer pattern. The hydraulic coupling in an automotive-type automatic transmission provides a good example of a hydrodynamic power transmission system.

Hydrostatic SystemsLiquid contained within an enclosed conductor is moved and pressurized by a positive-displacement pumping mechanism in a hydrostatic-type system. The energy implanted by the pump mechanism on the liquid is then available to move a linear hydraulic cylinder for push or pull action or to a rotary hydraulic motor for rotary force and motion. Pumps can be structured accomplish the desired liquid movement with several different basic design characteristics.

Major Hydraulic Components

Types of Hydraulic CircuitsThere are generally two types of hydraulic systems:1. Open-Centre System2. Closed-Centre SystemOpen-Centre System

Department of Mechanical Engineering UIET

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In this system, a control-valve spool must be open in the center to allow pump flow to pass through the valve and return to the reservoir. To operate several functions simultaneously, an open-centre system must have the correct connections; an open-centre system is efficient on single functions but is limited with multiple functions.

This type of system is commonly used with fixed displacement pumps and occasionally with variable pumps. The load may be a cylinder, a motor or any other hydraulic device. This type of system is said to be load-sensitive. That is, the pump delivers only the pressure required to move the load. An open hydraulic circuit contains at least one pump supplied with liquid from a tank or a reservoir, usually at atmospheric pressure. The discharge of the pump or pumps is directed through appropriate valves to the hydraulic cylinder or motor thereby providing the desired linear or rotary force and motion.

Closed-Centre System


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In this system, a pump can rest when the oil is not required to operate a function. This means that a control valve is closed in the center, stopping the flow of the oil from the pump.

The closed-centre system cuts off pump flow when the valve is in the neutral position. In these conditions, the pump flow is directed through the relief valve, which can be particularly wasteful of input energy as well as generating considerable fluid heating. Thus this particular system would normally only be used with a pressure compensated pump where the output is automatically reduced to zero when pressure increases to a preset level. The actual energy loss from the pump operating in these conditions can then be quite low. A variable displacement pump is generally used in this circuit.

Graphic Symbols


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Hydraulic Graphic Symbols are used in place of actual component drawings because they are easier to read and quick to draw. The symbols are of a standard recognized worldwide.

Lines and Line Functions

Pumps

Motors

Actuators

Valves


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Hydraulic PumpsEvery hydraulic system uses one or more pumps to pressurize the hydraulic fluid. The fluid under pressure, in turn, performs work in the output section of the hydraulic system. Thus, the pressurized fluid may be used to move a piston in a cylinder or to turn the shaft of a hydraulic motor.

Types of PumpsThree types of pumps find use in hydraulic system:1. Positive Displacement Pumps2. Negative Displacement Pumps, and

Positive Displacement Pumps

With these pumps, a definite volume of liquid is delivered for each cycle of pump operation, regardless of resistance, as long as the capacity of the power unit driving a pump is not exceeded. If an outlet is completely closed, either the unit driving a pump will stall or something will break. Therefore, a positive-


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displacement-type pump requires a pressure regulator or pressure-relief valve in the system. These pumps can be classified as follows:

1. Fixed Displacement Pumps2. Variable Displacement Pumps

Fixed Displacement Pumps

Gear Pumps

Gear Pump

Gear Pumps are further of two types:

1. External Gear Pumps2. Internal gear Pumps

External Gear Pump


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The external gear pump is capable of developing higher fluid pressures than a vane pump and can also be run at higher speeds.

Figure shows the operating principle of an external gear pump.

It consists of a driving gear and a driven gear enclosed in a closely fitted housing. The gears rotate in opposite directions and mesh at a point in the housing between the inlet and outlet ports.

As the teeth of the two gears separate, a partial vacuum forms and draws liquid through an inlet port into chamber A. Liquid in chamber A is trapped between the teeth of the two gears and the housing so that it is carried through two separate paths around to chamber B. As the teeth again mesh, they produce a force that drives a liquid through an outlet port.

Internal Gear Pumps

Certain use is made of internal gear pumps for hydraulic services. One internal gear is located within an outer gear ring, the tooth form being chosen so that each tip of each internal gear is always in contact with the inner surface of the outer ring. Rotation produces a series of contracting and expanding pockets transferring oil from the inlet side to the outlet side.


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The teeth of one gear project outward, while the teeth of the other gear project inward toward the center of the pump.

The two gears mesh on one side of a pump chamber, between an inlet and the discharge. On the opposite side of the chamber, a crescent-shaped form stands in the space between the two gears to provide a close tolerance.

The rotation of the internal gear by a shaft causes the external gear to rotate.

Since the two are in mesh. Everything in the chamber rotates except the crescent, causing a liquid to be trapped in the gear spaces as they pass the crescent.

Liquid is carried from an inlet to the discharge, where it is forced out of a pump by the gears meshing. As liquid is carried away from an inlet side of a pump, the pressure is diminished, and liquid is forced in from the supply source.

The size of the crescent that separates the internal and external gears determines the volume delivery of this pump. A small crescent allows more volume of a liquid per revolution than a larger crescent.


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Variable Displacement Pumps

Vane Pumps

Vane Pumps are particularly suited to medium-pressure, medium-speed duties and hence have the particular advantage over gear pumps that the rotor can be hydraulically balanced, thus minimizing bearing loads. Their main application is for low and medium-pressure systems requiring a compact low cost pump (eg machine tool hydraulic systems), their versatility being an attractive feature.

Unbalanced Vane Pump

Unbalanced design, (as shown in figure), a cam ring's shape is a true circle that is on a different centerline from a rotor's.

• Pump displacement depends on how far a rotor and ring are eccentric.

• The advantage of a true-circle ring is that control can be applied to vary the eccentricity and thus vary the displacement.

• A disadvantage is that an unbalanced pressure at the outlet is effective against a small area of the rotor's edge, imposing side loads on the shaft.


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Balanced Vane Pump

In the balanced design (as shown in figure), a pump has a stationary, elliptical cam ring and two sets of internal ports.

• A pumping chamber is formed between any two vanes twice in each revolution.

• The two inlets and outlets are 180 degrees apart.

• Back pressures against the edges of a rotor cancel each other.

• Recent design improvements that allow high operating speeds and pressures have made this pump the most universal in the mobile-equipment field.

Piston Pumps

Piston pumps offer high volumetric efficiencies together with virtually no limit on capacity, and thus covering a wide range of delivery requirements. Because of the greater complexity of construction, however, they are seldom competitive in smaller sizes with gear or vane pumps, unless high system pressures are required. In this respect they


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are superior to all other types of pump, although the pressure rating of a piston pump is governed by the types of the valve which can be employed with the design. Piston pumps are either radial or axial.

Radial Piston Pumps

In a radial piston pump, the pistons are arranged like wheel spokes in a short cylindrical block.

A drive shaft, which is inside a circular housing, rotates a cylinder block. The block turns on a stationary pintle that contains the inlet and outlet ports.

As a cylinder block turns, centrifugal force slings the pistons, which follow a circular housing.

Housing’s centerline is offset from a cylinder block's centerline. The amount of eccentricity between the two determines a piston stroke and, therefore, a pump's displacement.


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Controls can be applied to change housing's location and thereby vary a pump's delivery from zero to maximum.

Axial Piston Pumps

In hydraulic systems with a workingpressure above aprox. 250 bar the most used pumptype is the axial piston pump.

The pistons move parallel to the axis of the drive shaft. The swashplate is driven by the shaft and the angle of the swashplate determines the stroke of the piston.

The valves are necessary to direct the flow in the right direction. This type of pump can be driven in both directions but cannot be used as a hydromotor.

The animation shows how the displacement of an axial piston pump can be adjusted. In this example we use an axial piston pump with a rotating cylinder barrel and a static' swashplate.

The cylinder barrel is driven by the drive shaft which is guided through a hole in the swashplate. The position (angle) of the swashplate determines the stroke of the pistons and therefore the amount of displacement (cm3/omw) of the pump.


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By adjusting the position of the swashplate the amount of displacement can be changed. The more the swashplate turns to the vertical position, the more the amount of displacement decreases.

In the vertical position the displacement is zero. In that case the pump may be driven but will not deliver any oil. Normally the swashplate is adjusted by a hydraulic cylinder built inside the pumphousing.

Hydraulic Actuators A hydraulic actuator receives pressure energy and converts it to

mechanical force and motion. An actuator can be linear or rotary. A linear actuator gives force and motion outputs in a straight line. It is

more commonly called a cylinder but is also referred to as a ram, reciprocating motor, or linear motor.

A rotary actuator produces torque and rotating motion. It is more commonly called a hydraulic motor or motor

Hydraulic Motors Hydraulic motors convert hydraulic energy into mechanical energy.


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In industrial hydraulic circuits, pumps and motors are normally combined with a proper valving and piping to form a hydraulic-powered transmission.

A pump, which is mechanically linked to a prime mover, draws fluid from a reservoir and forces it to a motor. A motor, which is mechanically linked to the workload, is actuated by this flow so that motion or torque, or both, are conveyed to the work.Figure shows the basic operations of a hydraulic motor.

The main types of motors are gear, vane, and piston. They can be unidirectional or reversible. (Most motors designed for mobile equipment are reversible.)

External Gear Motor Radial Piston/Cam Motor


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Hydraulic Motor Applications:

Compact and extremely efficient, small hydraulic motors can be used for various machining operations like boring, reaming, drilling etc. Due to their small size they are tools of choice for applications like:

Electric motor coil winding

Oil pipeline inspection equipment

Undersea camera manipulation

Jumbo jet maintenance jacks

Milling and sawing applications

Dynamite blast hole pump drive

Automatic clamping

Textile washing agitators

Orange peeling machines

Fan drives

Diamond wheel dresser

Drill and tap machine tool

Chicken processing machinery

Conveyor drives

Cylinders A cylinder is a hydraulic actuator that is constructed of a piston or

plunger that operates in a cylindrical housing by the action of liquid under pressure.

Cylinder housing is a tube in which a plunger (piston) operates.


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Figure shows the basic parts of a cylinder. In a ram-type cylinder, a ram actuates a load directly. In a piston

cylinder, a piston rod is connected to a piston to actuate a load. An end of a cylinder from which a rod or plunger protrudes is a rod end.

The opposite end is a head end. The hydraulic connections are a head-end port and a rod-end port (fluid supply).

Single-Acting Cylinder. This cylinder (above Figure) only has a head-end port and is operated hydraulically in one direction.

• When oil is pumped into a port, it pushes on a plunger, thus extending it. To return or retract a cylinder, oil must be released to a reservoir. A plunger returns either because of the weight of a load or from some mechanical force such as a spring.

• In mobile equipment, flow to and from a single-acting cylinder is controlled by a reversing directional valve of a single-acting type.

Double-Acting Cylinder. This cylinder must have ports at the head and rod ends.

Pumping oil into the head end moves a piston to extend a rod while any oil in the rod end is pushed out and returned to a reservoir.

To retract a rod, flow is reversed. Oil from a pump goes into a rod end, and a head-end port is connected to allow return flow.

The flow direction to and from a double-acting cylinder can be controlled by a double-acting directional valve or by actuating a control of a reversible pump.


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Double-Acting Cylinder

Valves

Valves are used in hydraulic systems to control the operation of the actuators.

Valves regulate pressure by creating special pressure conditions and by controlling how much oil will flow in portions of a circuit and where it will go.

The three categories of hydraulic valves are pressure-control, flow-control, and directional-control.

Some valves have multiple functions, placing them into more than one category.

Valves are rated by their size, pressure capabilities, and pressure drop/flow.

Pressure-Control Valves

Pressure level control will fall into families associated with the function to be performed. We can divide these categories into six basic families and several subfamilies.

1. Relief and/or Safety Valves, first, limit the maximum system pressure which, in turn, protects the system components, piping, and tubing; and second. Limit the maximum output force of the hydraulic system.


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Relief Valve2. Sequence Valves are used to assure that one operation has been

completed before another function is performed. They operate by isolating the secondary circuit from the primary circuit until the set pressure is achieved.

3. Counter balance, over centre, holding, or brake valves are a broad range of pressure valves which controls a load induced pressure to hold and control the motion of the load. This group of valves provides balancing forces which prevent the load from running away because of its own weight or because of inertia.

Counterbalance Valve4. Unloading Valves are usually used in circuits with two or more pumps or

in circuits incorporating accumulators. The valve operates by sensing pressure in the system downstream of a check valve. Once a certain pressure level is obtained, the unloading valve unloads its pump to tank.


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Relief, Unloading and Check combination5. Reducing Valves are used to limit a certain branch of the hydraulic circuit

to a pressure lower than the relief valve setting for the rest of the system. By reducing pressure in the secondary circuit, we can independently limit the output force to that in the primary circuit.

Reducing Valve

1 Control Spool 6 Poppet2 Orifice 7 Relief Valve Poppet3 Orifice 8 Spring


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4 Orifice 9 Fluid Channel5 Flow past poppet 10 Pilot Section

Flow Control ValvesFlow-control valves are used to control an actuator's speed by metering flow. Metering is measuring or regulating the flow rate to or from an actuator. The simplest form of a restrictor or throttling valve is a unit incorporating an orifice. If restriction is required in both directions this need be nothing more than a simple orifice plate incorporated in a suitable fitting.

Flow Control ValveFor one-way restriction a spring-loaded poppet valve may be used, with the orifice drilled through the valve or a fixed orifice with a spring-loaded tapered needle. The latter has the advantage of rendering the restriction characteristics independent of the fluid viscosity. Alternatively, the needle may be made adjustable, so that the valve can be adjusted to provide different throttling characteristics. There are many alternative designs, including multi-orifice restrictors, screw and plunger, but needle and orifice configurations are the more usual.

Directional Control Valves (DCV)


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Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. Directional Control Valves provide links between various parts of the hydraulic system, by connecting of disconnecting and/or changing the oil flow direction. There are various types of DCV; in hydraulics the spool valve type is most common.

Spool Valve type DCV for OLSS circuit

Spool Valve type DCV for CLSS circuit

Hydraulic Excavators

Introduction


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The hydraulic excavators are the earth movers playing a major role in the development of the infrastructure like excavation, road construction, building construction, granite mining, ore mining, coal mining etc. The hydraulic excavators are the machines, which are powered by the hydraulic power for the earth excavation. The fabricated components of machinery are under carriage, revolving frame, boom, arm, and bucket.

Earth Moving MachineDefinition: It is a self propelled or towed machine on wheels, crawler of legs having equipment or attachment (working tool) primarily designed to perform excavating, loading, transporting, spreading, compacting or trenching of earth, rock and similar material.

According to IS: 12138-1987 an excavator is a self-propelled crawler or wheeled machine with an upper structure capable of minimum 360 degrees rotation which excavates, swings and discharges, material by the action of the bucket fitted to the boom and arm or telescopic boom without moving the chassis or undercarriage during any part of the working cycle of the machine. Excavator is a multipurpose earthmoving machine, which can perform many duties in the field.


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Such as digging earth, mining, and loading quarrying apart from other activities like well-digging, material handling. The excavator is the only earthmoving machine capable of working in all directions.

Operating PrincipleMechanical energy is transmitted from the engine (prime mover- electric motor or diesel engine) to hydraulic energy by the pumps. The flow of the oil generated by the pump is passed through the control valves to the output devices such as hydraulic cylinders (which cause relative movements of boom, arm and bucket with respect to each other and to the upper structure) and hydraulic motors (for swing of the upper structure with respect to the undercarriage and travel of the equipment on the ground and for the other auxiliary functions).Special hydraulic circuits are incorporated which give optimum use of hydraulic energy and have safety feature for components thereby reducing losses to minimum.

ExcavatorExcavators are heavy equipment used in civil works and surface mining. An excavator, also called a 360-degree excavator or digger, sometimes abbreviated simply to a 360, is an engineering vehicle consisting of a backhoe and cab mounted on a pivot (turntable is a more apt description) atop an undercarriage with tracks or wheels. The term excavator is sometimes used as a general term for any piece of digging equipment.

Types of Hydraulic Excavators


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1. Crawler ExcavatorApplications:1. Used in soft terrain- more mobility and stability2. Heavy excavation-quarry and mining operation

3. Where there is no need for frequent or high speed movement of the machine

4. Where terrain is uneven or sharp rock segment

2. Wheeled Excavator

Applications:

1. Rapid job-job mobility is important

2. Wherever crawler traction is impractical like legal Regulations Work floor damage


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Demand for speed and mobility

Nomenclature of hydraulic excavator (crawler type)

Base Machine, crawler excavator

1 Undercarriage chassis 6 Revolving frame2 Swing bearing 7 Track assembly3 Upper structure 8 Track Pad4 Cabin 9 Sprocket5 Counterweight 10 Idler


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Loader- General Nomenclature

1 Boom pivot 5 Arm cylinder2 Arm pivot 6 Bucket pivot3 Arm 7 Bucket4 Boom cylinder 8 Bucket cylinder

Excavators often have attachments, or you can purchase additional attachments to fit the machinery. A few of the attachments include jackhammers (breakers), shovels, grapples, augers, etc. Grapples are similar to claws and are used to grasp objects (trees, stumps, etc.). Hydraulic Mining Excavators often uses shovels. Finally, augers are similar to a drill bit, and are used to move materials.

The role of excavators is to dig holes, trenches, and foundations. Excavators use large machinery with hinge metal buckets, which are often attached to hydraulic arms, while using the equipment to move heavy or bulky quantities of soil or earth.


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Classification of Hydraulic Excavator is done based on1. Bucket capacity: Volumetric capacity of bucket.2. Operating weight: Total dead weight of machine in “tons”.3. Rated horsepower: of the engine in kW/HP4. Ground pressure: force/unit area of contact machine exerted on

ground by the machine.5. Swing speed: maximum attainable revolving speed of the upper

structure in rev/min.6. Travel speed: in km/hr.7. Grade ability: maximum ability of machine to climb a slope expressed

in percentage.8. Boom length: straight-line length between centres of hinge points of

the boom.9. Arm length: straight-line length between centres of hinge points of

the arms.10.Break out force: force exerted at the tip of the bucket due to

hydraulic thrust of the cylinder, which tends to move the bucket against resistance.

11. Arm crowd force: force exerted at the tip of the bucket due to hydraulic thrust of the cylinder, which moves the arm.

12. Hydraulic pumps: can be fixed displacement or variable flow pump. Amongst fixed displacement gear or axial piston. Variable displacement is axial piston or radial piston.

13. Operating pressure: maximum attainable rate of flow generated by a hydraulic circuit, which is determined by the setting of relief valve.

14. Flow rate: maximum attainable rate of flow generated by a hydraulic pump, which is attained at the minimum hydraulic pressure in the circuit.

15. Digging reach: maximum reach of the excavator with a particular attachment that is expressed as the distance from the centre of the machine to the tip of maximum extended attachment in the horizontal plane.


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16.Digging depth: maximum distance below the level ground that the tip of the attachment can reach.

17. Dumping height: maximum height at which the bucket can dump the prior filled material.

18. Cutting height: maximum height up to, which the attachment can cut or dig in the vertical plane.

19. Lifting capacity: maximum capacity of the machine to carry material with bucket in the curled position and both boom & arm fully extended.

Applications of excavators

1. Below ground level Applications Canal excavation Trench digging Pipe laying Burrow pit excavation Land levelling

2. Below/above ground level Applications Well sinking Dredging Handling of loose material

3. Above ground level Applications (with shovel attachment- bottom/forward dumping)

Mining Quarrying Bulk earthmoving against face Tunnelling4. Above ground level Applications (with special attachment)


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Rock breaking, Demolition Wood handling General construction- Vibratory pile drivers, excavators Scrap handling


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CLSSFeatures

CLSS stands for Closed Centre Load Sensing System and is featured as follows:

Fine controllability without affect of load. Controllability that allows digging even in the fine control mode. Ease of compound operation in which the flow distribution performance

depends on spool opening area during compound operation. Saving of energy by variable pump control. Better fuel economy. Less system heat.

Configuration

The CLSS consists of a variable displacement piston pump, a control valve and actuators.

The pump body consists of a main pump, a PC valve and an LS valve.

(courtesy L&T-Komatsu)


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Load Sensing System in CLSS circuit

Load Sensing System is a hydraulic system that senses and provides only the pressure and flow required by the hydraulic system. The components required to accomplish the characteristics of the load sensing system are: A variable volume piston pump, which has a compensator that will allow the pump to standby at low pressure when the system is not being actuated. It will sense the flow requirements of the system when it is being actuated and provide a variable flow rate as the flow demands of the hydraulic system are varied. The pump must also sense and respond to the varying pressure requirements of the hydraulic system. Most hydraulic systems do not operate at constant pressure. The hydraulic pressure will vary as the load on the hydraulic system changes. A control valve, with special sensor passages and checks, is also required to get the full benefit of the load sensing system. When the hydraulic system is not being operated, and is in the standby mode, the control valve must cut off the pressure signal from the actuating cylinder (or motor) to the pump. This causes the pump to automatically go into low pressure standby when the system is not being operated. When the control valve is actuated, the control valve will pick up the pressure requirement from the actuating cylinder (or motor) and send that pressure signal back to the pump where the pump starts to respond to the system pressure. The flow requirement of the system is dictated by the movement of the spool. The system flow requirement is sent back to the pump, through the signal line, from the control valve. This combination of a load sensing pump and load sensing control valve allows the total system to provide only the flow and pressure required by the load sensing system. This system automatically adjusts to the varying pressure and flow demands. It remains in high pressure standby until the load is overcome or the valve spool is returned to neutral. It produces only enough flow to make up for internal leakage.


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Basic Principle of CLSS1. Control of Pump Swash Plate Angle The pump swash plate angle (pump delivery) is controlled so that the LS

differential pressure ∆PLS, which is the difference between the pump discharge pressure PP and the LS pressure PLS (actuator load pressure) at the control valve outlet, becomes constant.(LS differential pressure ∆PLS = Pump pressure PP – LS pressure PLS)



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When the LS differential pressure ∆PLS reduces below the set pressure of the LS valve (when the actuator load pressure is high), the pump swash plate angle will move in the direction of maximum. When the set pressure is raised (when the actuator load pressure is low), the pump swash plate angle will move in the direction of minimum.



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2. Pressure Compensation Control

A valve (pressure compensation valve) is mounted on the outlet side of the control valve. In case of compound operation of the actuator with this valve, the differential pressure ∆P between the spool upstream (inlet) and the downstream (outlet) of each valve becomes constant irrespective of load (pressure). So, the flow from the pump is distributed (compensated) in proportion to the opening area S1 and S2 of each valve being operated.



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3. LS Valve


The LS valve detects loads and controls delivery. This valve controls main pump delivery Q with differential pressure ∆PLS

(PP – PLS) [that is called LS differential pressure] between the main pump pressure PP and control valve outlet pressure PLS.

This valve is applied with main pump pressure PP, pressure PLS that is obtained from constant valve output [that is called LS Pressure] and pressure PSIG from the LS-EPC valve [that is called LS Selection Pressure].

The relations of differential pressure ∆PLS (= PP – PLS) between main pump pressure PP and LS pressure PLS with delivery Q vary with the LS selection current ISIG of the LS-EPC valve as shown in figure.

As ISIG changes from 0 to 1A, the spring set force changes accordingly, and the selector point for pump discharge amount changes from 0.64 to 2.1 MPa {6.5 to 21.5 kg/cm²} at the standard median.


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4. PC Valve


When pump discharge pressure PP rises, the control valve spool stroke will increase and the opening area will enlarge. So, the PC valve controls pump delivery Q so that delivery Q does not increase above a certain level depending on discharge pressure PP.The valve also controls the pump absorbing hydraulic horsepower to approximately equal horsepower so that the pump absorbing horsepower does not exceed the engine horsepower.


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This means that, when a load to the actuator increases during operation and pump discharge pressure PP rises, this valve will reduce pump delivery Q, or when pump discharge pressure PP drops, this valve will increase delivery Q.

In this case, the relations between pump discharge pressure PP and pump delivery Q change as shown in the figure since the current value given to the PC-EPC valve solenoid is regarded as a parameter.

However, some PC valves have the function to sense actual engine speeds in the heavy-duty operation mode and to reduce pump delivery and recover speed when the speed reduces due to increase of load.

In other words, when an increase of load reduces engine speed below the set value, the command current from the controller to the PC-EPC valve solenoid will increase as engine speed reduces and will reduce the pump swash plate angle.


5. Control Valve


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The control valve is a CLOSED-SPOOL system. However the hydraulic pump does go to minimum flow. UNLOAD valves open to permit oil to return to tank when in neutral.


This control valve consists of 6 spool valves and one service valve.Since one spool of this control valve is used for one work equipment unit, the structure is simple.


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Spool Valve

Hydraulic Spool Working of a Spool Valve

A hydraulic spool valve is a switching device used to control hydraulic devices. A spool valve can turn the flow of hydraulic fluid from a hydraulic pump to an actuator on and off by blocking off the route the fluid takes. It is a cylinder inside a sealed case. It usually has valves leading to the pump and the tank on one side, and valves leading to one or more hydraulic devices on the other side. Pressure can flow into the valve from the pump into the hydraulic devices, or drain out of them back into a hydraulic storage tank. A controller moves the valve back and forth in its case to slide the spools into different positions. The position of the rotor will only allow the hydraulic fluid to flow in one direction to perform a specific task.

In closed centre type when the engine or motor is started, pump flow enters the directional control valve, but because it is a closed centre type, the flow is blocked. A control valve is equipped with special sensor passages and checks to get the full benefit of the load sensing system. When the hydraulic system is not being operated, and is in the standby mode, the control valve must cut off the pressure signal from the actuating cylinder (or motor) to the pump. This causes the pump to automatically go into low pressure standby when the system is not being operated. When the control valve is actuated, the control valve picks up the pressure requirement from the actuating cylinder (or motor) and sends that pressure signal back to the pump where the pump starts to respond to the system pressure. The flow requirement of the system is dictated by the movement of the spool. The system flow requirement is sent back to the pump, through the signal line, from the control valve.


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Unload Valve1. When the control valve is neutralFUNCTION• When the control valve is neutral, the delivery Q equivalent to the pump minimum swash plate angle is released to the tank circuit. At the time, pump discharge pressure PP is set to 2.45 MPa {25.0kg/cm2} with spring (3) inside the vale. (The LS pressure PLS is 0 MPa {0kg/cm2}.)

(Courtesy L&T-Komatsu)1. Unload Valve PLS: LS circuit (pressure)2. Sleeve PP: Pump circuit (pressure)3. Spring T: Tank circuit4. Spool

OPERATION• Pump discharge pressure PP is applied to the left end face of spool (4) and LS pressure PLS is applied to the right end face.• Since the LS pressure PLS is 0 when the control valve is neutral, pump discharge pressure PP is only applied and is set with the lead to spring (3).• When pump discharge pressure PP rises to spring (3) load (2.45 MPa {25.0 kg/cm2}), spool (4) will move toward the right side and pump circuit PP will interconnect to tank circuit T through the drill hole.


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• Therefore, pump discharge pressure PP is set to 2.45 MPa {25.0 kg/cm2}.

2. When the control valve is in the fine control modeFUNCTION• When the control valve is in the fine control mode and the requested flow of the actuator is less than the pump minimum swash plate angle, pump discharge pressure PP is set to LS pressure PLS + 2.45 MPa {25.0 kg/cm2}.When the differential pressure between discharge pressure PP and LS pressure PLS comes to spring (3) load (2.45 MPa {25.0 kg/ cm2}), the unload valve will open and LS differential pressure ∆PLS will come to 2.45 MPa {25.0 kg/cm2}.

(Courtesy L&T-Komatsu)1. Unload Valve PLS: LS circuit (pressure)2. Sleeve PP: Pump circuit (pressure) 3. Spring T: Tank circuit 4. Spool

OPERATION• When the control valve is operated in the fine control mode, LS pressure PLS will occur and will be applied to the right end face of spool (4). At the time


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differential pressure between LS pressure PLS and pump discharge pressure PP increases because the opening area of the control valve spool is small.• When the differential pressure between pump discharge pressure PP and LS pressure PLS comes to spring (3) load (2.45 MPa {25.0 kg/ cm2}), spool (4) will move to the right side and pump circuit PP will interconnect to tank circuit T.• This means that pump discharge pressure PP is set to the spring force (2.45 MPa {25.0 kg/cm2} + LS pressure PLS, and LS differential pressure ∆PLS comes to 2.45 MPa {25.0 kg/cm2}.

3. When the control valve is operatedFUNCTION• If the required flow of the actuator increases over the pump minimum swash plate angle when the control valve is operated, the flow to tank circuit T will be interrupted and pump delivery Q will be completely flown to the actuator circuit.

(Courtesy L&T-Komatsu)1. Unload Valve PLS: LS circuit (pressure)2. Sleeve PP: Pump circuit (pressure)3. Spring T: Tank circuit4. Spool


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OPERATION• When the control valve is operated with large stroke, LS pressure PLS will occur and will be applied to the right end face of spool (4). At the time, the opening areas of the control valve spool are large and the difference between LS pressure PLS and pump discharge pressure PP is small.• So, the differential pressure between pump discharge pressure PP and LS pressure PLS does not reach spring (3) load (2.45 MPa {25.0 kg/ cm2}) and spring (3) pushes spool (4) to the left side.• Then, pump circuit PP and tank circuit T are interrupted, and pump delivery Q is completely flown to the actuator circuit.

Travel Valve

(Courtesy L&T-Komatsu)

OPERATION


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• When spool (1) is operated, the pump discharge pressure PP will be led to actuator circuit A through bridge passage b from flow control valve (2) and spool notch a.• At the same time, the actuator circuit pressure PA moves pressure reducing valve (3) to the right side, and notches c and d interconnect to the travel junction circuit e and LS circuit PLS respectively.• So, actuator circuit pressure PA (= A) is led from notch c to LS circuit PLS through notch d.

The travel circuit is different from the work equipment circuit, actuator circuit pressure PA is directly led to the LS circuit PLS.

5. Self Pressure Reducing Valve


P1: From pump 1. Control Valve blockPR: Supply to solenoid valve, 2. Valve (sequence valve)PPC valve and EPC valve. 3. SpringT: To hydraulic tank


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1. Screw 8. Valve(pressure reducing valve)2. Poppet 9. Spring (safety valve)3. Spring (pressure reducing valve pilot) 10. Ball4. Spring (pressure reducing valve main) 11. Filter

The self pressure reducing valves reduces the discharge pressure of the main pump and supplies it to the solenoid valve, the PPC valve, etc. as the control pressure.


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7. PPC Valve


P: From main pump P3: Left: Swing RIGHT/Right: Bucket Curl

P1: Left: Arm OUT/Right: Boom LOWER P4: Left: Swing LEFT/Right: Bucket Dump

P2: Left: Arm IN/Right: Boom RAISE T: To tank


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1. Spool 6. Nut (for linking the lever)2. Metering spring 7. Joint3. Centring spring 8. Plate4. Piston 9. Retainer5. Disk 10. Body



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Operation1) At neutral• Ports A and B of the control valve and ports P1 and P2 of the PPC valve are connected to drain chamber D through fine control hole f in spool (1). (Fig. 1)


2) During fine control (neutral→ fine control)When piston (4) starts to be pushed by disc (5), retainer (9) is pushed; spool (1) is also pushed by metering spring (2), and moves down. When this happens, fine control hole f is shut off from drain chamber D, and at almost the same time, it is connected to pump pressure chamber PP, so pilot pressure oil from the main pump passes through fine control hole f and goes from port P1 to port A. When the pressure at port P1 becomes higher, spool (1) is pushed back and fine control hole f is shut off from pump pressure chamber PP. At almost


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the same time, it is connected to drain chamber D to release the pressure at port P1.When this happens, spool (1) moves up or down so that force of metering spring (2) is balanced with the pressure at port P1. The relationship in the position of spool (1) and body (10) (fine control hole f is at a point midway between drain hole D and pump pressure chamber PP) does not change until retainer (9) contacts spool (1). Therefore, metering spring (2) is compressed proportionally to the amount of movement of the control lever, so the pressure at port P1 also rises in proportion to the travel of the control lever. In this way, the control valve spool moves to a position where the pressure in chamber A (the same as the pressure at port P1) and the force of the control valve spool return spring are balanced. (Fig. 2)



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3) During fine control (when control lever is returned)When disc (5) starts to be returned, spool (1) is pushed up by the force of centring spring (3) and the pressure at port P1.When this happens, fine control hole f is connected to drain chamber D and the pressure oil at port P1 is released. If the pressure at port P1 drops too far, spool (1) is pushed down by metering spring (2), and fine control hole f is shut off from drain chamber D. At almost the same time, it is connected to pump pressure chamber PP, and the pump pressure is supplied until the pressure at port P1 recovers to a pressure that corresponds to the lever position. When the spool of the control valve returns, oil in drain chamber D flows fine control hole f' in the valve on the side that is not working. The oil passes through port P2 and enters chamber B to fill the chamber with oil. (Fig. 3)



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4) At full strokeWhen disc (5) pushes down piston (4), and retainer (9) pushes down spool (1), fine control hole f is shut off from drain chamber D, and is connected with pump pressure chamber PP. Therefore, the pilot pressure oil from the main pump passes through fine control hole f and flows to chamber A from port P1, and pushes the control valve spool. The oil returning from chamber B passes from port P2 through fine control hole f' and flows to drain chamber D. (Fig. 4)



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8. Engine Control

(courtesy L&T-Komatsu) 1. Starting switch 2. Fuel control dial 3. Governor motor 4. Starting motor 5. Linkage 6. Battery relay 7. Battery 8. Engine throttle and pump controller 9. Fuel injection pump

The engine can be started and stopped with only starting switch. The engine throttle and pump controller receives the signal of fuel control dial and transmits the drive signal to governor motor to control the governor lever angle of fuel injection pump and control the engine speed.


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1. OPERATION OF SYSTEMStarting engine• When the starting switch is turned to the START position, the starting signal flows to the starting motor, and the starting motor turns to start the engine. When this happens, the engine throttle and pump controller checks the signal from the fuel control dial and sets the engine speed to the speed set by the fuel control dial.


Engine speed control• The fuel control dial sends a signal to the engine throttle and pump controller according to the position of the dial. The engine throttle and pump controller calculates the angle of the governor motor according to this signal, and sends a signal to drive the governor motor so that it is at that angle. When this happens, the operating angle of the governor motor is detected by the potentiometer, and feedback is sent to the engine throttle and pump controller, so that it can observe the operation of the governor motor.


Stopping engine


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• When the starting switch is turned to the STOP position, the engine throttle and pump controller drives the governor motor so that the governor lever is set to the NO INJECTION position.• When this happens, to maintain the electric power in the system until the engine stops completely, the engine throttle and pump controller itself drives the battery relay.


Fuel Control Dial

FUNCTION• The fuel control dial is installed under the monitor panel, and a potentiometer is installed under the knob. The potentiometer shaft is turned by turning the knob.


• As the potentiometer shaft is turned, the resistance of the variable resistor in the potentiometer changes and a throttle signal is sent to the engine throttle


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and pump controller. The hatched area in the graph shown at right is the abnormality detection area.

Governor Motor

(courtesy L&T-Komatsu)1. Potentiometer2. Cover3. Shaft4. Dust seal5. Bearing6. Motor7. Gear8. Connector

OPERATIONWhile motor is stopped• Electric power is applied to both phases A and B of the motor.


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While motor is running• The engine throttle and pump controller supplies a pulse current to phase A and B, and the motor revolves, synchronizing to the pulse.

FUNCTION• The motor is turned according to the drive signal from the engine throttle and pump controller to control the governor lever of the fuel injection pump.This motor used as the motive power source is a stepping motor.• A potentiometer for feedback is installed to monitor the operation of the motor.• Revolution of the motor is transmitted.

9. Electric Control System CONTROL FUNCTION



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Engine and Pump Control Function

FUNCTION• This function is for selecting any of the four working modes "A," "B", "E" and "L" with the working mode selector switch on the monitor panel. The controller can select optimum engine torques or pump absorption torques for works to be expected.



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• The controller detects the engine governor speed set with the fuel control dial depending on the pump absorption torque set in each mode and detects actual engine speeds. Then, the controller controls all torques at each output point of the engine so that the pump can absorb them.


• When an engine speed was lowered, the controller prevents the engine from stopping by throttling the pump absorption toque.


1. CONTROL METHOD IN EACH MODEMode A• Matching point in Mode A

A Travel (A1) 66.2 KW/2,200 rpm{88.7 HP/2,200 rpm}

A Work (A2) 65.5 KW/2,200 rpm{87.8 HP/2,200 rpm}

• When a load to the pump increases and the pressure rises and the engine speed lowers. At the time, the controller reduces the pump delivery so that the


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speed lowers to the speed at the full output point or so. If the pressure drops on the contrary, the controller increases the pump delivery so that the speed comes to the speed at the full output point or so. The controller repeats these controls so that the engine can always be used at speeds at the full output point or so.

(courtesy L&T-Komatsu)Mode E/ Mode B/ Mode L

Mode E B LPartial Output Point

90%

90%

55%

Model PC-130-7Mode E 58.8 kW/2,000 rpm

{78.9 HP/2,000 rpm}Mode B 58.8 kW/2,000 rpm

{78.9 HP/2,000 rpm}Mode L 36.8 kW/1,500 rpm

{70 HP/1,500 rpm}


• At this time, the controller keeps the pump absorption torque along the constant horsepower curve and lowers the engine speed by the composite control of the engine and pump.• By this method, the engine is used in the low fuel consumption area.


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2. FUNCTION TO CONTROL PUMP DURING TRAVEL• If the machine travels in mode work A, B, E, or L, the working mode does not change, but the pump absorption torque and engine speed rise to travel A mode.


2. PUMP/VALVE CONTROL FUNCTION


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FUNCTION


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• The machine is matched to various types of work properly with the 2-stage relief function to increase the digging force, etc.1) Cut-off function• When the cut-off function is turned on, the PC-EPC current is increased to near the maximum value. By this operation, the flow rate in the relief state is lowered to reduce fuel consumption.• Operating condition for turning on cut-off function.Condition- The average value of the front and rear pressure sensors is above 27.9 MPa {285 kg/cm2} and the one-touch power maximizing function is not turned on.The cut-off function does not work, however, while the machine is travelling in mode A, the lock switch is turned on.


2) 2-stage relief function• The relief pressure in the normal work is 31.9 MPa {325 kg/cm2}. If the 2 stage relief function is turned on, however, the relief pressure rises to about 34.8 MPa {355 kg/cm2}. By this operation, the hydraulic force is increased further.• Operating condition for turning on 2-stage relief functionCondition Relief Pressure• During travel• When swing lock switch isturned on• When one-touch power maximizingfunction is turned on• When L mode is operated

31.9 MPa{325 kg/cm2} ↓34.8 MPa{355 kg/cm2}

3. ONE-TOUCH POWER MAXIMIZING FUNCTION


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FUNCTION• Power can be increased for about 8.5 sec. By operating the left knob switch.


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1) One-touch power maximizing function• When the operator needs more digging force to dig up a large rock, etc., if the left knob switch is pressed, the hydraulic force is increased about 9% to increase the digging force.• If the left knob switch is turned on in working mode "A" or "E", each function is set automatically as shown below.

Working Mode Engine/Pump Control

2-stage relief function

Operation Time

A, E Matching at ratedoutput point

31.9 MPa {325 kg/cm2} ↓34.8 MPa {355 kg/cm2}

Automaticallyreset at 8.5 sec

4. AUTO-DECELERATION FUNCTION

FUNCTION


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• If the all control levers are set in NEUTRAL while waiting for a dump truck or work, the engine speed is lowered to the medium level automatically to reduce the fuel consumption and noise.• If any lever is operated, the engine speed rises to the set level instantly.

OPERATIONWhen control levers are set in neutral• If all the control levers are set in NEUTRAL while the engine speed is above the decelerator operation level (about 1,400 rpm), the engine speed lowers instantly to the first deceleration level about 100 rpm lower than the set speed.• If 4 more seconds pass, the engine speed lowers to the second deceleration level (about 1,400 rpm) and keeps at that level until any lever is operated again.When any control lever is operated• If any control lever is operated while the engine speed is kept at the second deceleration level, the engine speed rises instantly.



Date post:	07-Apr-2015
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